Luminosity Profiles of Merger Remnants

Using published luminosity and molecular gas profiles of the late-stage mergers NGC 3921, NGC 7252 and Arp 220, we examine the expected luminosity profiles of the evolved merger remnants, especially in light of the massive CO complexes that are observed in their nuclei. For NGC 3921 and NGC 7252 we predict that the resulting luminosity profiles will be characterized by an r^{1/4} law. In view of previous optical work on these systems, it seems likely that they will evolve into normal ellipticals as regards their optical properties. Due to a much higher central molecular column density, Arp 220 might not evolve such a ``seamless'' light profile. We conclude that ultraluminous infrared mergers such as Arp 220 either evolve into ellipticals with anomalous luminosity profiles, or do not produce many low-mass stars out of their molecular gas complexes.Comment: Final refereed version. Note new title. 4 pages, 2 encapsulated color figures, uses emulateapj.sty. Accepted to ApJL. Also available at http://www.cv.nrao.edu/~jhibbard/Remnants/remnants.htm

Radio and Far-Infrared Emission as Tracers of Star Formation and AGN in Nearby Cluster Galaxies

We have studied the radio and far-infrared (FIR) emission from 114 galaxies in the 7 nearest clusters (<100 Mpc) with prominent X-ray emission to investigate the impact of the cluster environment on the star formation and AGN activity in the member galaxies. The X-ray selection criterion is adopted to focus on the most massive and dynamically relaxed clusters. A large majority of cluster galaxies show an excess in radio emission over that predicted from the radio-FIR correlation, the fraction of sources with radio excess increases toward cluster cores, and the radial gradient in the FIR/radio flux ratio is a result of radio enhancement. Of the radio-excess sources, 70% are early-type galaxies and the same fraction host an AGN. The galaxy density drops by a factor of 10 from the composite cluster center out to 1.5 Mpc, yet galaxies show no change in FIR properties over this region, and show no indication of mass segregation. We have examined in detail the physical mechanisms that might impact the FIR and radio emission of cluster galaxies. While collisional heating of dust may be important for galaxies in cluster centers, it appears to have a negligible effect on the observed FIR emission for our sample galaxies. The correlations between radio and FIR luminosity and radius could be explained by magnetic compression from thermal ICM pressure. We also find that simple delayed harassment cannot fully account for the observed radio, FIR, and mid-IR properties of cluster galaxies.Comment: 21 pages, 15 figures, Accepted by Ap

Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

(abridged) We investigate the detailed response of gas to the formation of transient and long-lived dynamical structures induced in the early stages of a disk-disk collision, and identify observational signatures of radial gas inflow through a detailed examination of the collision simulation of an equal mass bulge dominated galaxy. Stars respond to the tidal interaction by forming both transient arms and long lived m=2 bars, but the gas response is more transient, flowing directly toward the central regions within about 10^8 years after the initial collision. The rate of inflow declines when more than half of the total gas supply reaches the inner few kpc, where the gas forms a dense nuclear ring inside the stellar bar. The average gas inflow rate to the central 1.8 kpc is \~7 Msun/yr with a peak rate of 17 Msun/yr. The evolution of gas in a bulgeless progenitor galaxy is also discussed, and a possible link to the ``chain galaxy'' population observed at high redshifts is inferred. The evolution of the structural parameters (the asymmetry and concentration) of both stars and gas are studied in detail. Further, a new structural parameter (the compactness parameter K) that traces the evolution of the size scale of the gas relative to the stellar disk is introduced. Non-circular gas kinematics driven by the perturbation of the non-axisymmetric structure can produce distinct emission features in the "forbidden velocity quadrants'' of the position-velocity diagram (PVD). The dynamical mass calculated using the rotation curve derived from fitting the emission envelope of the PVD can determine the true mass to within 20% to 40%. The evolution of the molecular fraction $M_H2/M_(H2 + HI) and the compactness (K) are potential tracers to quantitatively assign the age of the interaction.Comment: 52 pages, 20 figures (9 jpgs), accepted for publication in ApJ Version with all figures at http://cfa-www.harvard.edu/~diono/ms.ps.g

Color Magnitude Relation and Morphology of Low-Redshift ULIRGs in SDSS

We present color-magnitude and morphological analysis of 54 low-redshift ULIRGs, a subset of the IRAS 1Jy sample (Kim & Sanders, 1998), in the SDSS. The ULIRGs are on average 1 magnitude brighter in M0.1r than the SDSS galaxies within the same redshift range. The majority of the ULIRGs (~87%) have the colors typical of the blue cloud, and only 4 sources (~7%) are located in the red sequence. While ULIRGs are popularly thought to be precursors to a QSO phase, we find few (~6%) in the "green valley" where the majority of the X-ray and IR selected AGNs are found, and none of which harbors an AGN. For the 14 previously spectroscopic identified AGNs (~28%), we perform PSF subtractions and find that on average the central point sources contribute less than one third to the total luminosity, and that their high optical luminosities and overall blue colors are apparently the result of star formation activity of the host galaxies. Visual inspection of the SDSS images reveals a wide range of disturbed morphologies. A detailed morphology analysis using Gini and M20 coefficients shows that slightly less than one half (~42% in g band) of the ULIRGs are located in the region where most local mergers are found. The heterogeneous distribution of ULIRGs in the G-M20 space is qualitatively consistent with the results found by numerical simulations of disk-disk mergers. Our study also shows that the measured morphological parameters are systematically affected by the SNR and thus the merging galaxies can appear at various regions in the G-M20 space. In general, our results reinforce the view that ULIRGs contain young stellar populations and are mergers in progress. Our study provides a uniform comparison sample for studying ULIRGs at higher redshifts such as Spitzer mid-IR selected ULIRGs at z=1~2 and submm galaxies.Comment: 42 pages, 11 figures, ApJ accepte

Detection of CO (2-1) and Radio Continuum Emission from the z = 4.4 QSO BRI 1335-0417

We have detected redshifted CO (2-1) emission at 43 GHz and radio continuum emission at 1.47 and 4.86 GHz from the z = 4.4 QSO BRI 1335-0417 using the Very Large Array. The CO data imply optically thick emission from warm (>30 K) molecular gas with a total mass, M(H_2), of 1.5+/-0.3 x10^{11} M_solar, using the Galactic gas mass-to-CO luminosity conversion factor. We set an upper limit to the CO source size of 1.1", and a lower limit of 0.23"x(T_ex/50K)^{-1/2}, where T_ex is the gas excitation temperature. We derive an upper limit to the dynamical mass of 2x10^{10} x sin^{-2} i M_solar, where i is the disk inclination angle. To reconcile the gas mass with the dynamical mass requires either a nearly face-on disk (i < 25deg), or a gas mass-to-CO luminosity conversion factor significantly lower than the Galactic value. The spectral energy distribution from the radio to the rest-frame infrared of BRI 1335-0417 is consistent with that expected from a nuclear starburst galaxy, with an implied massive star formation rate of 2300+/-600 M_solar yr^{-1}.Comment: standard AAS LATEX forma

Molecular Gas Content of HI Monsters and Implications to Cold Gas Content Evolution in Galaxies

We present 12CO (J=1-0) observations of a sample of local galaxies (0.04<z<0.08) with a large neutral hydrogen reservoir, or "HI monsters". The data were obtained using the Redshift Search Receiver on the FCRAO 14 m telescope. The sample consists of 20 HI-massive galaxies with M(HI)>3e10Msun from the ALFALFA survey and 8 LSBs with a comparable M(HI) (>1.5e10Msun). Our sample selection is purely based on the amount of neutral hydrogen, thereby providing a chance to study how atomic and molecular gas relate to each other in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA selected galaxies and 4 out of 8 LSBs with molecular gas mass M(H2) of (1-11)e9Msun. Their total cold gas masses of (2-7e10Msun make them some of the most gas-massive galaxies identified to date in the Local Universe. Observed trends associated with HI, H2, and stellar properties of the HI massive galaxies and the field comparison sample are analyzed in the context of theoretical models of galaxy cold gas content and evolution, and the importance of total gas content and improved recipes for handling spatially differentiated behaviors of disk and halo gas are identified as potential areas of improvement for the modeling.Comment: 18 pages, 11 figures, 2 tables; Accepted for publication in MNRA

DETECTION OF DIFFUSE NEUTRAL INTRAGROUP MEDIUM IN HICKSON COMPACT GROUPS

We present new Green Bank Telescope (GBT) 21 cm neutral hydrogen (H I) observations of a complete distance-limited sample of 22 Hickson Compact Groups (HCGs) with at least four true members. We detected an average H I mass of 8 × 109 M (median = 6 × 109 M ), which is significantly larger than previous single-dish measurements. Consequently, the H I deficiencies for these HCGs have been reduced, although not completely eliminated. Spectral comparison of the GBT data with complementary Very Large Array data shows significant H I excess in the GBT spectra. The observed excess is primarily due to the high surface brightness (HSB) sensitivity of the GBT detecting diffuse, low column density H I in these groups. The excess gas forms a faint diffused neutral medium which is an intermediate stage in the evolution of HSB H I tidal debris in the intragroup medium (IGM) before it is fully ionized. The excess gas mass fraction, (M(H I)GBT – M(H I)VLA)/M(H I)GBT, for our complete sample varies from 5% to 81% with an average of 36% (median = 30%). The excess gas mass fraction is highest in slightly H I deficient groups where the tidal debris has had enough time to evolve. We also find the excess gas content increases with the evolutionary phase of the group described in Verdes-Montenegro et al. Theoretical calculations indicate that an H I cloud of radius ≥ 200 pc would survive in an IGM of 2 × 106 K for more than the typical dynamical lifetime of a group. However, smaller clouds get evaporated and assimilated into the hot IGM in a much shorter timescale

Neutral Gas Properties and Lyα\alpha Escape in Extreme Green Pea Galaxies

Mechanisms regulating the escape of Ly$\alpha$ photons and ionizing radiation remain poorly understood. To study these processes we analyze VLA 21cm observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and HST COS spectra of 17 GP galaxies at $z<0.2$. All are highly ionized: J1608 has the highest [O III] $\lambda5007$/[O II] $\lambda3727$ for star-forming galaxies in SDSS, and the 17 GPs have [O III]/[O II] $\geq6.6$. We set an upper limit on J1608's HI mass of $\log M_{HI}/M_\odot=8.4$, near or below average compared to similar mass dwarf galaxies. In the COS sample, eight GPs show Ly$\alpha$ absorption components, six of which also have Ly$\alpha$ emission. The HI column densities derived from Ly$\alpha$ absorption are high, $\log N_{HI}/$cm$^{-2}=19-21$, well above the LyC optically thick limit. Using low-ionization absorption lines, we measure covering fractions (f_{\mbox{cov}}) of $0.1-1$, and find that f_{\mbox{cov}} strongly anti-correlates with Ly$\alpha$ escape fraction. Low covering fractions may facilitate Ly$\alpha$ and LyC escape through dense neutral regions. GPs with f_{\mbox{cov}}\sim1 all have low neutral gas velocities, while GPs with lower f_{\mbox{cov}}=0.2-0.6 have a larger range of velocities. Conventional mechanical feedback may help establish low f_{\mbox{cov}} in some cases, whereas other processes may be important for GPs with low velocities. Finally, we compare f_{\mbox{cov}} with proposed indicators of LyC escape. Ionizing photon escape likely depends on a combination of neutral gas geometry and kinematics, complicating the use of emission-line diagnostics for identifying LyC emitters.Comment: 21 pages, 11 figures, accepted for publication in Ap